This kind of so-called Rosen type piezoelectric transformer is made by providing primary and secondary electrodes on a piezoelectric ceramic such as PZT and polarizing them respectively by high electric fields. When a voltage at a natural resonance frequency which is determined from a length direction is applied to the primary side, the element vibrates by an inverse piezoelectric effect, and it is possible to take out from a secondary side a voltage according to vibration by a piezoelectric effect.
A piezoelectric body can be obtained by applying a high electric field along a certain direction of a ceramic to align the crystal axes. Then, among piezoelectric bodies, when a tension is applied some generate positive charges in a positive direction of a coordinate axis of the force (a sign of piezo-electricity is positive), while others generate negative charges (a sign of piezo-electricity is negative).
FIG. 4 is an explanatory diagram of polarizations of secondary Rosen type piezoelectric transformers. When polarization directions (the arrows show the polarization directions) in secondary sides or primary sides of piezoelectric transformers made from a same material are reversed and a voltage at a resonance frequency is applied to the primary sides, electric potentials of opposite signs occur in the secondary sides respectively.
In the diagram, the polarization direction of the primary electrodes 2a and 2b of a piezoelectric transformer 2 is one which is reversed from the polarization direction of the primary electrodes 1a and 1b of a piezoelectric transformer 1. When a voltage at a resonance frequency is applied to the primary electrodes 1a and 1b and the same voltage is applied to the primary electrodes 2a and 2b, electric potentials of opposite signs respectively occur in the secondary electrodes 1c and 2c. 
The polarization direction of the secondary electrode 3c of the piezoelectric transformer 3 is one which is reversed from a polarization direction of a secondary electrode 1c of the piezoelectric transformer 1. When a voltage at a resonance frequency is applied to the primary electrodes 1a and 1b and the same voltage is applied to the primary electrodes 3a and 3b, electric potentials of opposite signs occur respectively in the secondary electrodes 1c and 3c. 
FIG. 5 is an explanatory diagram of a piezoelectric transformer driving method in a conventional example (patent document 1: Japanese Patent Laid-Open No. 2000-307165). In the two piezoelectric transformers used, the primary electrodes 1a and 1b of the piezoelectric transformer 1 and the primary electrodes 2a and 2b of the piezoelectric transformer 2 have polarization directions reverse to each other, and the secondary electrode 1c of the piezoelectric transformer 1 and the secondary electrode 2c of the piezoelectric transformer 2 have a mutually same polarization direction.
A cold-cathode tube L is connected between the secondary electrode 1c of the piezoelectric transformer 1 and the secondary electrode 2c of the piezoelectric transformer 2. One terminal from an AC power supply E is connected to the primary electrode 1a of the piezoelectric transformer 1 and the primary electrode 2a of the piezoelectric transformer 2, and the other terminal from the AC power supply E is connected to the primary electrode 1b of the piezoelectric transformer 1 and the primary electrode 2b of the piezoelectric transformer 2. Thus, the piezoelectric transformer 1 and piezoelectric transformer 2 are connected in parallel in view of the AC power supply E.
Since a polarization direction of the primary electrodes 1a and 1b of the piezoelectric transformer 1 and a polarization direction of the primary electrodes 2a and 2b of the piezoelectric transformer 2 are reverse, when a voltage at a resonance frequency is applied to the primary electrodes 1a and 1b and the primary electrodes 2a and 2b, a large voltage is applied to the cold-cathode tube L connected between the secondary electrodes 1c and 2c. For example, when a positive voltage is outputted from the secondary electrode 1c of the piezoelectric transformer 1, a negative voltage in reverse polarity is outputted from the secondary electrode 2c of the piezoelectric transformer 2.
Nevertheless, in the driving method of the piezoelectric transformers connected in parallel in the above-described conventional example, a plurality of resonance points (resonance frequencies f1 and f2 in FIG. 6) occur as shown in a transmission characteristic (the boost ratio characteristic to the frequency output voltage) shown in FIG. 6(a). In addition, FIG. 6(b) is a wiring diagram at the time of transmission characteristic measurement of FIG. 6(a). A reason why the plurality of resonance points occur is mainly that there is a variation among respective piezoelectric transformers. In addition, for the purpose of reference, a transmission characteristic at the time of using one piezoelectric transformer is shown in FIG. 7(a), and a wiring diagram at the time of its measurement is shown in FIG. 7(b). In FIG. 7, a range of higher frequency than a resonance point is the region of use. In order to dissolve two or more resonance points shown in FIG. 6(a), it is necessary to make a pair of piezoelectric transformers of identical properties. However, there was a problem that it was not easy to make all the properties of the piezoelectric transformers same even if the same materials were used and the manufacturing processes were managed.
The present invention is proposed in view of the above-mentioned problems, and aims at providing a piezoelectric transformer driving apparatus and a piezoelectric transformer driving method where an unnecessary resonance point does not come out near a frequency to be used, that is, an output is stable to the frequencies even if there is a variation between the respective piezoelectric transformers.